SocketManager: restrict priority range

Previously SocketManager supported 4 billion priority levels, but we only ever
used a few of them. There are some optimizations I want to make that will be
simpler if we can restrict the number of possible priorities. This change
limits users to 5 named priorities.
Merge into master from pull request #120:
SocketManager: restrict priority range (https://github.com/floodlight/indigo/pull/120)